The present invention relates to a learning control method for synchronously controlling at a high precision the rotational angle of a main shaft (spindle) and the position of a feed shaft in an NC (numerical control) machine tool.
When the rotational angle of the main shaft is controlled for machining in synchronization with the position of the feed shaft by detecting the rotational angle of the main shaft of an NC lathe or the like with a pulse generator mounted on the main shaft, and by outputting the value designating the position of the feed shaft according to the pulse from the main shaft pulse generator, it is necessary to enhance the function of a servo system of the feed shaft in order to increase the machining precision. For instance, in order to reduce a follow-up error of the feed shaft which is controlled in position, it is necessary to increase a position loop gain of the feed shaft system. However, if the position loop gain is increased excessively, the control becomes unstable. There was heretofore a limitation in the reduction of the follow-up error on the feed shaft.
There has been proposed a method which takes not of the fact that the value designating the position of the feed shaft tends to be repeated cyclically during the above mentioned synchronized control, and uses the response in repetitive operations to enhance the follow-up accuracy of the feed shaft system in a learning mode.
FIG. 1 is a block diagram showing an embodiment of the NC apparatus which realizes the aforementioned learning control method and which includes a basic servo system and a repetitive controller 50. The basic servo system forms a stable closed loop including, as shown in FIG. 2, a subtracter S which subtracts a current position y(t) from a positional command value r(t), and a transfer element G.sub.o which positions the shaft at the current position y(t) by using the positional deviation value .epsilon.(t) from the subtracter S.
As shown in the block diagram shown in FIG. 1, the system includes a subtracter S which subtracts the current position y(t) from the positional command value r(t), an adder A1 which adds a corrected value c(t) obtained from a repetitive controller to the positional deviation value .epsilon.(t) obtained from the subtracter S, and a transfer element G.sub.o which commands the current position y(t) according to the corrected positional deviation value obtained from the adder A1. The repetitive controller 50 is composed of a positive feedback loop including an adder A2 which adds a deviation compensating value to the positional deviation .epsilon.(t) from the subtracter S, a deviation compensator Fe.sup.-sL including a filter F(s) which limits the band zone and a dead time element e.sup.-sL of a length equal to the repetition cycle L of the positional command value r(t), and a dynamic characteristic compensator Gx(s) which outputs a corrected value c(t) to the adder A1 based on the deviation compensating value obtained from the deviation compensator Fe.sup.-sL for stabilizing the control system and improving the characteristics. Refer to "Inoue et al.: High Precision Control in Repetitive Operation of Proton Synchrotron Electromagnetic Power Source. Proceedings of Denki-gakkai C-100.7 (1980), p. 234; Inoue et al.: High Precision Control of Playback-servo System. Proceedings of Denki-gakkai C-101.4 (1981), p. 89; Komata et al.: Extension of Repetitive Control to Multivariable Systems. Proceedings of Keisoku Jido-Seigyo Gakkai, Vol. 20, No. 9 (September, 1984); Sinnaka: Repetitive Control for Cyclic Time Change System. Proceedings of Denki-gakkai C-106.10. (1986), p. 209." This type of NC apparatus is used in practive, for instance, for cutting non-circular configurations. Refer to "Higuchi et al.: Study on Oil Pressure Servo. Proceedings of Spring Session of Seiki-gakkai 1983, p. 875; Higuchi et al.: Study on Non-circular Configuration Cutting. Proceeding of Autumn Session of Seiki-gakkai 1984, p. 305; Higuchi et al.; Study on Non-circular Configuration Cutting. Proceedings of Autumn Session of Seiki-gakkai 1984, p. 307; Higuchi et al.: Study on Non-circular Configuration Cutting. Proceedings of Spring Session of Seiki-gakkai 1985, p. 27."
However, the aforementioned learning control methods have the following defects as discussed below in paragraphs (1) to (3).
(1) The follow-up accuracy is theoretically increased by repeating learning operations (as shown in FIG. 3 with a solid line). But in practice, the follow-up accuracy sometimes deteriorates after it is improved once (as shown with a broken line in FIG. 3). When the follow-up accuracy deteriorates, the learning process may be suspended by a command using an absolute value of the follow-up accuracy. But as the absolute value of the follow-up accuracy differs depending on the shape of the positional path designated for the feed shaft, it is impossible to command the suspension of the above learning step. PA1 (2) If the learning for the designated path is performed at the same time as machining a work, the cycle time of the machining process inevitably increases. PA1 (3) Since the dynamic characteristic compensator of the repetitive controller which is used for the learning control differs depending on each NC machine tool, the effect of the learning control varies widely.